15N natural abundance of foliage and soil across boreal forests of Finland

This study presents the latitudinal variation (from 60 degrees 30' N to 68 degrees 2' latitude) of natural abundances of N-15 in the foliage, humus and soils of boreal forests of Finland. Our results clearly showed that N concentration of the foliage did not change significantly with latitudes but their N-15 values were significantly higher in higher latitude sites relative to that of the mid and lower latitude sites, indicating the different forms of N uptake at the higher latitudes compared to the lower latitudes. We assume that the higher foliage delta N-15 values of the higher latitudes trees might be due to either more openness of N cycle (greater proportional N losses) in these latitudes compared to the sites of southern latitudes (lower N losses) or the differences in their mycorrhizal associations. Regression analysis showed that the temperature was the main factor influencing the N-15 natural abundance of humus and soil of all forest ecosystems, both before and after clear-cut, whereas rainfall was the main controlling factor to the foliage N-15. Possible reasons behind the increasing delta N-15 natural abundances of plants and soils with increasing latitudes are discussed in this paper. The clear-cut did not show any specific trend on the N-15 fractionation in humus and soil, i.e. both N-15-enrichment and -depletion occurred after clear-cut.